Mutual positions of Earth and external planets
External (or superior) planets are defined
those planets that are farther from the Sun, in comparison with the
Earth.
We speak of Mars, Jupiter, Saturn, Uranus, Neptune
and Pluto . Their positions in comparison with the Earth and the
Sun determine the conditions of visibility of it.
Mutual positions of Earth and external planets
In the case of external planets, their motion appears to take place in a clockwise direction in comparison with the Earth, because of their slower revolution around the Sun.
Position 1: it is said conjunction. The planet is invisible because it is perspectively lined up with the Sun. The planet is at its maximum distance from the Earth.
From position 1 to 2: in this period the planet begins to be visible in the morning, before the rising of the Sun. Its rising gradually anticipates.
Position 2: it is said quadrature. The planet is visible in the second part of the night, since it rises at the real midnight of the place from which it is observed. Its distance from the Earth is intermediate.
From position 2 to 3: in this period the planet rises in the evening, before midnight and it is visible up to dawn.
Position 3: it is said opposition. The planet reaches the least distance from the Earth. It is in the condition of optimal visibility, since, besides the least distance, it results visible for the whole night. It is in the opposite direction to the Sun, the planet rises in fact at the sunset of the Sun and it remains in the sky up to dawn.
From position 3 to 4: the planet already rises before the sunset of the Sun. It is visible in the evening and for great part of the night. The distance from the Earth starts to increase again.
Position 4: it is said of quadrature. The planet, at the sunset of the Sun, is at its transit. It is therefore visible for the first part of the night, setting at real midnight of the observation place.
From position 4 to 1: in this period the
setting of the planet takes always place earlier, up to when, in proximity
of a new conjunction, the planet is lost in the lights of the
sunset.
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Position
of internal planets
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